Hopes of a new ceasefire in the three-week-old Gaza war rose and then quickly fell on Tuesday afternoon when a senior official announced a 24-hour pause in fighting, saying he was speaking on behalf of Hamas and Islamic Jihad, the two main militant organisations – a statement quickly disavowed by Hamas.

Yasser Abed Rabbo, a veteran Palestinian negotiator and the secretary general of the Palestine Liberation Organisation, said the factions had agreed to a 24-hour ceasefire with the possibility of an extension. His announcement came shortly before a Palestinian delegation, including president Mahmoud Abbas and representatives of Hamas and Islamic Jihad, was due to meet in Cairo for talks aimed at ending the fighting.

The move came after Israel intensified its military operation in Gaza overnight. Among the targets hit was Gaza’s only power plant, where fuel tanks exploded causing huge black clouds of smoke. “The power plant is finished,” said its director, Mohammed al-Sharif, signalling a new crisis for the population, which was already enduring power cuts of more than 20 hours a day

Gaza endured a night of relentless bombardment that brought some of the heaviest pounding since the start of the conflict three weeks ago, in the hours after the Israeli political and military leadership warned of a protracted offensive.

Palestinian officials said at least 100 Palestinians had been killed in the coastal enclave so far on Tuesday.

Israeli forces focused on key strategic targets, including the home of the Hamas leader in Gaza, Ismail Haniyeh, and a building housing Hamas-controlled broadcast outlets.

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Haniyeh’s home was hit by a missile shortly before dawn, causing damage but no injuries. Eleven people were killed in a strike on a house in Bureij refugee camp in Gaza City.

The shells that hit fuel tanks at Gaza’s only power plant caused a massive explosion and black smoke to billow into the air. The plant’s capacity – already down to about three hours’ electricity supply a day – is likely to be further reduced.

Hamas said al-Aqsa TV and al-Aqsa Radio were also targeted. The television station continued to broadcast but the radio station went silent.

The Israel Defence Forces struck 150 targets in total during the course of the night. Sirens warning of rocket fire sounded across southern Israel.

The IDF said overnight that five soldiers had died in a gun battle on Monday with militants who crossed into Israel via a tunnel near the community of Nahal Oz, close to the border with the Gaza Strip.

The incident raised to 10 the number of military fatalities for the day, bringing the total to 53. Four others were killed in a mortar attack and another died in clashes in the south of Gaza.

Hamas’s armed wing, the Qassam Brigades, claimed it had killed 19 Israeli soldiers on Monday and a total of 110 during the military campaign.

Speaking at a televised press conference on Monday evening, the Israeli prime minister warned that the operation would be stepped up, in remarks that flouted international pressure for a ceasefire. “We need to be prepared for a protracted campaign. We will continue to act with force and discretion until our mission is accomplished,” said Binyamin Netanyahu.

The IDF continued to categorically deny that its forces were responsible for hits on Shati refugee camp and the Shifa hospital on Monday. At least eight children were killed at Shati while playing in a park.

The military released an aerial photograph that it said showed rockets fired by militants had fallen short. In a statement it said red lines drawn over the photograph indicated “the paths of the four terrorist rockets, as detected by IDF radars and sensors, that were launched in the attacks that resulted in one hitting the Al-Shifa hospital and one hitting the Shati refugee camp. Of the other two rockets, one landed at sea and the other was intercepted on its way to the city of Ashkelon.”

Witnesses in Gaza said missiles had been fired from Israeli F-16 jets. A spokesman for the interior ministry in Gaza, Iyad al-Buzm, said explosives experts from the Gaza police had examined “the targeted places and the remnants of shells there” as well as the wounds on the bodies, determining them to be from an Israeli strike.

The Palestinian death toll stood at about 1,200, mostly civilians. Two Israeli civilians and a Thai agricultural worker have been killed in rocket fire in the past three weeks.

The renewed bloodshed followed growing international calls for a ceasefire. On Monday the UN secretary general, Ban Ki-moon, reiterated the security council’s earlier call for an immediate and unconditional ceasefire, demanding that both Israel and Hamas end the fighting “in the name of humanity”.

The US president, Barack Obama, told Netanyahu by phone on Sunday of his concern at civilian casualties. He also pressed for an immediate ceasefire.

Meanwhile, there were fresh clashes in East Jerusalem between Palestinian protesters against the war in Gaza and Israeli security forces.

Monday evening’s statements from the three men directing the Israeli military offensive on Gaza – Netanyahu, defence minister Moshe Ya’alon and military chief of staff Benny Gantz – will gratify hawkish cabinet ministers and media commentators who have been stridently urging an expansion of the operation in order to deal a decisive blow to Hamas.